Various types of equipment are used to provide hard surfaces for streets, highways, parking lots, etc. Included among that array of equipment is an asphalt paver, which utilizes a screed to place a layer or mat of asphalt material on an underlying subgrade. Preferably, asphalt paving has a substantially planar surface in order to provide a smooth ride for vehicles subsequently passing thereover. Thus, other than perhaps for following the gradual curvature of the underlying terrain and for intentional "crowning" for encouraging drainage of surface water from the finished surface, the mat placed by the paver has a substantially planar surface. After the paver places a mat of asphalt material on the subgrade, a heavy roller is used to compact the asphalt material in order to provide a durable, non-porous surface. Ideally, the underlying subgrade also has a correspondingly substantially planar surface.
After the mat is placed by the paver, the mat is compacted with a heavy roller, which compresses the asphalt material to a factor of the thickness of the mat as laid by the paver. If the asphalt material has a uniform density and thickness, which is greater than a certain minimum thickness relative to the size of the aggregate contained in the asphalt material, then the actual thickness of the asphalt mat after compaction depends on the thickness of the asphalt material prior to compaction by the roller. The ratio between (a) the difference in thickness of the mat before and after compaction with the roller, and (b) the thickness of the asphalt mat as placed, is commonly referred to as the "compaction factor".
If the underlying subgrade and the asphalt material mat are both planar and if the asphalt material has a uniform density, then the rolled surface will also be planar, as desired. In an actual situation, however, the surface of the underlying subgrade generally has depressions and elevations that cause the surface of the compacted mat to vary substantially from a planar profile. Thus, the asphalt material mat, even though having a substantially planar surface as laid by the asphalt paver, is thicker is some places than in others. As a result, the asphalt, after compaction, no longer exhibits the substantially planar surface but, instead, has depressions and elevations similar to, but less pronounced than, those of the subgrade surface. This uneven result is sometimes referred to as "differential compaction".
For example, assume that the desired thickness of asphalt material nominally laid by a paver prior to compaction is six inches. Assume also that the subgrade has a local depression that is two inches deep and a ridge or local elevation that is two inches high. Thus, the thickness of the asphalt material laid by the paver would be eight inches deep over the local depression and only four inches deep over the local elevation. Assume further that the roller compacts the asphalt material to seventy-five percent of its original thickness as laid by the paver, or a reduction in thickness of twenty-five percent. After compaction by the roller, the thickness of the asphalt material over the substantially planar surface of the subgrade would be four and one-half inches.
Similarly, the thickness of the compacted asphalt material over the depression and the localized elevation would be six inches and three inches, respectively. In other words, the surface of the asphalt mat that was substantially planar, as provided by the paver prior to compaction by a roller, now has a surface over the depression that lies one-half inch below the surface of the nominal mat. Further, the surface of the compacted asphalt mat over the local elevation lies one-half inch above the surface of the compacted nominal mat and one-inch above the surface of the compacted mat above the depression. Such a situation obviously does not provide a smooth ride for a vehicle passing thereover.
In an attempt to compensate for such undesirable surface irregularities, many prior art pavers utilize a grade reference system, typically having a length of thirty to fifty feet and generally referred to as a "ski" or "averaging ski", wherein the surface deviations in the underlying subgrade in the direction of travel of the paver, sometimes referred to as longitudinal surface deviations, are averaged over the length of the ski.
Although most of the descriptions herein refer to the use of an averaging ski, it should be understood that a stringline or an existing surface, such as an abutting layer of asphalt paving for "joint matching", may be used in place of an averaging ski and the operating principle remains basically the same.
The averaging ski may be multi-footed, i.e., have several supporting feet gliding along and bearing generally against the underlying subgrade to establish an average reference for the nominal depth of asphalt material to be deposited thereon. In fact, dynamic positioning of the reference surface of the averaging ski may largely depend on the two highest relative points of the subgrade which two of the feet bear against at any given time.
Due to the leveling action of the screed in combination with the averaging ski, the paver can lay a relatively uniform mat over a subgrade having longitudinal deviations with periods on the order of, or greater than, the length of the averaging ski. Minimal perturbations, such as those arising from an exposed rock in the subgrade, can sometimes be compensated for by the spring loading of individual shoes supporting the averaging ski.
Unfortunately, however, the effects of many of the longitudinal subgrade deviations have periods that are less than the length of the averaging ski and, therefore, cannot be removed by use of prior art averaging skis. In other words, due to differential compaction, many of the localized deviations may be reduced in magnitude but, nevertheless, are still present after compaction of asphalt laid by a paver utilizing a prior art averaging ski.
A common practice currently utilized to minimize the effects of localized deviations is to place one or more leveling courses, or "lifts", to remove the low spots, or to use cold milling to remove the high spots. In either case, the goal is to lessen or remove the deviations before placing the topping or finishing surface mat of asphalt paving material. In other words, each successive layer more closely approximates the ideal subgrade.
What is needed, therefore, is an apparatus and method which takes proper account of differential compaction when placing asphalt paving material and which thereby reduces or eliminates the extra leveling courses normally required to remove the effects of localized deviations in an asphalt paving subgrade.